EXOSAT observations of the X-ray source in M15

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Abstract
We present the results from two long EXOSAT observations of the X-ray source in the globular cluster M15 (NGC 7078) totalling approximately 1 day and separated by 100 day. Variability is detected on time-scales down to 250 s, but no variations attributable to an eclipse were observed which is surprising in view of the large fluctuations recently reported by Auriere for his optical candidate AC211. There is no evidence for periodic or quasi-periodic structure in the data. The spectrum is similar in shape to that of many low-mass X-ray binaries, and is best fitted by a two-component model consisting of thermal bremsstrahlung or a power law and a low-temperature blackbody with low energy absorption. The absorption is significantly different between our two observations and shows an overall correlation with luminosity, contrary to the Einstein Observatory results reported by Hertz & Grindlay. We discuss these results in the context of other low-mass X-ray binaries. The lack of X-ray bursts is attributed to the low metallicity of M15.

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